1. Field of the Invention
The present invention relates to a vacuum casting apparatus and method wherein a molding cavity is reduced in pressure to a vacuum and upon opening a gate, a molten metal is charged from a molten metal retaining dome into the molding cavity at a high speed. More particularly, the present invention relates to a vacuum casting apparatus and method wherein the molten metal charged into the molding cavity is pressurized before it is solidified and during pressurization the molten metal is prevented from flowing backward toward the molten metal retaining dome.
2. Description of Related Art
As one example of a vacuum casting apparatus and method capable of producing cast products of a high quality and at a low cost, a casting apparatus and method wherein a molding cavity is reduced in pressure substantially to a vacuum and then a molten metal is charged into the molding cavity was proposed by the present applicant in Japanese Patent Publication HEI 2-155557.
The proposed casting apparatus includes a molding cavity which can be reduced in pressure substantially to a vacuum, a molten metal supply passage, a gate piston for opening and closing a passage connecting the molten metal supply passage to the molding cavity, and a pressure pin assembled in the gate piston. In the casting apparatus, the molding cavity is reduced in pressure to a vacuum. Then, the gate piston is opened so that a molten metal is charged through the molten metal supply passage into the molding cavity at a high speed. The gate is then closed to isolate the molding cavity filled with the molten metal from the molten metal retaining dome, and then the pressure pin is operated to pressurize the molten metal in the molding cavity, to solidify the molten metal quickly. In the molding, since the molding cavity is in a vacuum, involvement of air by the molten metal is prevented, so that casting defects due to bubbles are avoided. Further, since the charging speed is high, running of the molten metal in the molding cavity is improved and a production of slimmer casting products is possible.
However, the following problems yet remain in the above-described vacuum casting apparatus and method:
When the gate piston is closed to isolate the molding cavity from the molten metal retaining dome, a clearance inevitably remains between the molding die and the gate piston moved relative to the die. Further, pressurization of the molten metal in the molding cavity must be initiated within a short period of time after the gate piston has been closed, because if the pressurization occurs too late the molten metal would be allowed to solidify and a sufficient pressurization would not produce advantageous results. Due to these reasons, when the molten metal in the molding cavity is pressurized by the pressure pin, a portion of the molten metal will flow backward toward the molten metal retaining dome to cause an insufficient pressurization and molding defects.